Microscopic examination of unirradiated CANDU fuel from Darlington Nuclear GS

Between 2010 and 2014, Darlington NGS experienced an increase in the number of fuel defects. Elements from unirradiated CANDU fuel bundles manufactured in 2012 and 2013 were disassembled, inspected, and compared with elements manufactured circa 2002. These inspections revealed fractured fuel pellets, debris, and internal damage to the fuel sheaths that may have been caused during pellet loading. The fractured fuel pellets and damaged sheaths were examined in detail to characterize the material damage mechanisms and investigate the underlying causes. Differences in fuel pellet fracture modes were related to variations in UO₂ microstructure, which is typically controlled by the UO₂ powder feedstock, die compaction and sintering process parameters. The OPG investigation identified that the majority of fuel defects were confined to two manufacturing clusters. The focus of this paper is on the benefits of microscopic examinations of fresh fuel. (author)